02 July 2010

Drosophila spp.: Guinea Pigs with Wings

Sun filtered down through the trees, rustling in a steady wind: wonderful weather for humans, [David] Foote said, but terrible for flies. "These are the worst possible conditions—I just want to warn you." Of all the Hawaiian insects, the dearest to Foote is the family Drosophilidae, the pomace flies, better known, not entirely correctly, as fruit flies. Included in the family Drosophilidae is the genus Drosophila. Among genetics researchers, Drosophila is the organism of choice. Its chromosomes are few and big and easily extracted from its salivary glands; it reproduces at ten days of age, so genetic changes unfold observably from generation to generation, which means you can wrap up an experiment in a matter of weeks. Also, flies don't eat much—not like mice or lab rats, which will quickly chew a hole in one's departmental budget. Over the past century, the science of genetics has grown up around one drosophila species in particular, Drosophila melanogaster, a tiny red-eyed orange fly so ubiquitous that scientists simply shrug and call it "cosmopolitan." Drosophila melanogaster is a guinea pig with wings. It has been scrutinized and forcibly mutated, crossbred, back-bred, inbred. Scientists have created drosophilas with extra-long life spans (sixty days, instead of thirty), drosophilas with superior maze-navigating abilities, drosophilas dumb as posts; drosophilas with no legs, with legs sprouting from their heads, even—I saw their photograph a few years ago on the front page of The New York Times—with extra, ectopic eyes peering out from where their knees should be. A great deal of what we know about ourselves has been gleaned from monkeying with this pale orange fly.

In contrast, the drosophilas of Hawaii owe their oddity entirely to the whims of natural selection. They are a tribe unto themselves: oversize, with elaborate stripes and colorations and strange and intricate mating rituals. To prove their sexual worth, males of the species Drosophila heteroneura butt heads, which are elongated like those of hammerhead sharks. Males of the closely related Drosophila silvestris stand on their hind legs and grapple like boxers in the clinch. If you want to understand the genetics of colonization, isolation, and speciation in a nonlaboratory setting, the Hawaiian drosophilas are your subject; they are honeycreepers for entomologists. There are some six hundred species of Drosophila in Hawaii, one-fifth of all the Drosophila species in the world—the progeny of flies that tumbled from earlier Hawaiian islands and have been doing so for forty-two million years, ever since the one fly from which they are all descended blew in from somewhere else, to an island that long ago submerged. From one strange and alluring species to the next, drosophila are themselves a sort of archipelago of biodiversity.

And as surely as the honeycreepers are indicators of environmental change in Hawaii, so too are the drosophilas, perhaps more so. Ecologists sometimes describe an ecosystem as a sort of pyramidal hotel of energy consumers, built up of successive trophic layers of feeders and fed-upons: plants, which draw their energy from light; grazers, which draw their energy from plants and span a range of organisms from leaf-mining insects to fruit-eating bats to Jersey cows; and predators, like tigers, feral cats, Tyrannosaurus rex, and bird-eating brown tree snakes. It is a loose schema, with numerous exceptions and outstanding questions. (Which story, for example, do carnivorous army ants inhabit in the Amazonian rain forest pyramid?) The drosophilas occupy a janitoral closet near the base of this building. They subsist largely on decaying plant material-bark, branches, leaf litter. They are composters, thriving on the senescence and misfortune of their fellow hotel guests. Most everything ends with them. The drosophilas are so ubiquitous in Hawaiian rain forests and their microhabitats so varied that their seemingly minor fates in fact closely reflect the spectrum of disruptions and alterations unfolding above and around them-including but not limited to the damage caused, or said to be caused, by feral pigs. If your quarry is the pig, it pays to follow the flies.

"The drosophilas are decomposers in this ecosystem," Foote explained to me one afternoon on the Big Island. "They're responsible for breaking down the organic matter in plants and allowing the nutrients to be cycled up into the forest again. These particular species breed on plants that are some of the most sensitive to disturbance by pigs, cattle, and rats. So we're very concerned about their status."